1. Field of the Invention
This invention relates to a process for thermochemically transforming biomass or other oxygenated feedstocks into high quality liquid hydrocarbon fuels.
2. Description of Related Art
Oxygenated feedstocks, such as solid biomass (wood, agricultural waste, waste paper, etc.) can be converted into liquid products via rapid heating in the absence of oxygen (pyrolysis). A solid char product (consisting mostly of carbon, but also containing any non-volatile, inert compounds found in the feedstock) and non-condensable vapors (such as CO2 and CH4) are produced, along with condensable species such as: water, hydrocarbons, and molecules that contain carbon atoms, hydrogen atoms, and oxygen atoms. The proportions of the resulting products obtained depends on the rate of heating of the feedstock particles, as described by Mohan, et al. (Mohan, Pittman, and Steele, “Pyrolysis of Wood/Biomass for Bio-oil: A Critical Review,” in Energy & fuels, Volume 20, pp. 848-889, 2006). A type of biomass pyrolysis, referred to as “fast pyrolysis,” minimizes the amount of char produced, and maximizes the amount of condensable liquid obtained, by heating the biomass as rapidly as possible. Some char is always produced, particularly since biomass always contains some non-volatile, non-reactive compounds (generally referred to as ash). Conventional pyrolysis of biomass, typically fast pyrolysis, does not utilize or require gaseous hydrogen or catalysts and produces a dense, acidic, reactive liquid product that contains water, oils, and char formed during the process. Because fast pyrolysis is most typically carried out in an inert atmosphere, much of the oxygen present in biomass is carried over into the liquid products obtained, which increases their chemical reactivity. The liquids from fast pyrolysis also contain high levels of acids (such as acetic acid), as well as olefins and polyaromatic hydrocarbons. The chemically unstable liquids produced by conventional pyrolysis tend to thicken over time and can also react to a point where hydrophilic and hydrophobic phases form. Dilution of pyrolysis liquids with methanol or other alcohols has been shown to reduce the subsequent activity and viscosity of the oils, but this approach is not considered to be practically or economically viable, because large amounts of unrecoverable alcohol would be required to stabilize large amounts of pyrolysis liquids for transport and subsequent use.
In conventional pyrolysis of biomass, carried out in an inert environment, the water-miscible liquid product is highly oxygenated and reactive, for example, with total acid numbers (TAN) in the range of 100-200, has low chemical stability for polymerization, is incompatible with petroleum hydrocarbons due to inherent water miscibility and very high oxygen content (on the order of about 40% by weight), and has a low heating value. As a result, transport and utilization of this product are problematic and it is difficult to upgrade this product to a liquid fuel due to retrograde reactions that typically occur in conventional pyrolysis and in conventional fast pyrolysis. Upgrading technologies, as applied to conventional pyrolysis liquids, tend to yield only small quantities of deoxygenated high-quality liquid hydrocarbons that are suitable for use as transportation fuels.
In addition, the separation of char generated during conventional pyrolysis from the liquid pyrolysis product presents a technical challenge due to the large amounts of oxygen, olefins, acids, and free radicals in hot pyrolysis vapors which remain highly reactive and form a pitch-like material when they come in intimate contact with char particles on the surface of a barrier filter, inertial separation device, or electrostatic precipitator. In particular, barrier filters used to separate the char from the hot pyrolysis vapors (prior to cooling and condensation of the liquid pyrolysis products) can quickly experience irreversible clogging (blinding) due to the reactions of char and reactive vapors that occur on and within the layer of char on the surface of the filter.
In order to upgrade conventional pyrolysis liquids, attempts have been made to react the conventional pyrolysis liquids with hydrogen, in the presence of solid catalysts, in order to remove oxygen from the liquids and produce a stable, useful hydrocarbon product. This process is referred to as hydroconversion. However, the upgrading of conventional pyrolysis liquids via hydroconversion is commercially non-viable. Hydroconversion of conventional pyrolysis liquids consumes significant H2 at extreme process conditions, such as very high hydrogen pressures of 138 bar (2000 psig) or more. High specific pressures of hydrogen are required in order for the desired reactions to proceed, but these pressures create conditions wherein most of the oxygen removed from the liquid is removed via the formation of water (H2O). This approach consumes large amounts of hydrogen, thus making the process economically unattractive. In addition, hydroconversion reactors often plug due to accumulations of coke precursors present in the pyrolysis oils or from coke products resulting from catalysis. The coke is a solid product, consisting mostly of carbon, and the maintenance needed to remove it from hydroconversion reactors reduces further the economic viability of hydroconversion of conventional pyrolysis liquids.
The present state of the art also describes a different means by which oxygenated feedstocks such as biomass can be converted to create useful liquid hydrocarbons, referred to as hydropyrolysis. Hydropyrolysis can be carried out with or without the aid of a catalyst However, lower hydrocarbon yields and lower deoxygenation tend to be a characteristic of noncatalytic hydropyrolytic processes. Therefore, as described herein, “hydropyrolysis” will be considered to refer to a catalytic pyrolysis process carried out in the presence of molecular hydrogen (H2). Typically, the objective of conventional hydropyrolysis processes has been to remove heteroatoms (atoms other than carbon and hydrogen) from biomass, and maximize liquid hydrocarbon yield. In prior work by Meier, et al. (Meier, Jakobi and Faix, “Catalytic Hydroliquefaction of Spruce Wood,” in the Journal of Wood Chemistry and Technology, Vol. 8, No. 4, pp. 523-542, 1988), the solid biomass feedstock was processed in a reactor containing liquid, in which solid biomass feedstock was suspended. The reaction was carried out at high internal pressures of over 138 bar (2000 psig) with recycled slurry oil and the lowest oxygen content reported for hydrocarbons produced was 7.6% by mass. This value was obtained when a precious metal palladium (Pd) catalyst was used. In another study by Meier and Faix (Meier and Faix, “Solvent-Free Hydroliquefaction of Pine Wood and Miscanthus Stems,” in Proceedings of the International Conference on Biomass for Energy and Industry, Lisbon, Portugal, Oct. 9-13, 1989), in which a slurry oil was not used, the lowest oxygen content reported in the hydrocarbon product was 9.7% oxygen by mass, and the reaction was still carried out at high internal hydrogen pressures of over 138 bar (2000 psig) within a heated reactor with a NiMo catalyst.
In studies of single-stage hydropyrolysis of cellulose and other biomass-derived feedstocks, Rocha, et al. (Rocha, Luengo, and Snape, “The Scope for Generating Bio-Oils with Relatively Low Oxygen Contents via Hydropyrolysis,” in Organic Geochemistry, Vol. 30, pp. 1527-1534, 1999) demonstrated that, with a FeS catalyst, as the partial pressure of hydrogen in the hydropyrolysis reactor was decreased, the oxygen content of hydrocarbon product tended to increase. Experiments carried out at lower hydrogen pressures typically produced hydrocarbon products with oxygen contents above 15%. In one case described by Rocha, et al., cellulose was subjected to hydropyrolysis at a hydrogen pressure of 99 bar (1440 psig), and the lowest oxygen content of resulting hydrocarbon product was 11.5% by mass. Unfortunately, this approach compromises economy, as it requires an external source of H2 and must be carried out at high reactor pressures. In addition to requiring a continuous external input of hydrogen, such conventional hydropyrolysis processes produce excessive H2O which generally represents a waste stream. In this type of reactor, the hydropyrolysis of biomass has not been found to be economically attractive because the oxygen content of the hydrocarbon product was still fairly high after processing and the reaction conditions required by the process were too severe to be practical.
Finally, hydropyrolysis may be carried out in a fluidized bed (typically, a shallow fluidized bed with length:diameter ratio <1.5). However, the present invention pertains to means by which effective hydropyrolysis can be carried out in a single step in a deep fluidized bed of particles of an active catalyst, at H2 partial pressures from 200 to 600 psig, in such a manner that the oxygen content of the liquid hydrocarbon product is reduced to below 4% by mass. Also, in the present invention, the hydropyrolysis reaction is exothermic and provides the heat of reaction so that there is no need to provide external heating or circulate hot regenerated catalyst or sand through the fluid bed reactor as is typically required for traditional pyrolysis. Fluidized beds generally include solid particles, such as particles of sand or catalyst, that are agitated and fluidized by a stream of gas, which travels upward through the bed and exits from the bed at or near the top of the reactor. The behavior of fluidized beds is known to at least partially depend on the depth (or height, or length) of the bed. The bed depth is generally characterized by the L/D ratio, meaning the ratio of the depth, height, or length of the bed, divided by the bed diameter. The behavior of the bed will depend heavily on the particle size distribution of the material from which the bed is formed. Generally, fluidized beds are designed with an L/D of 1-2, since beds in this range exhibit uniform fluidization, once a flow rate of fluidizing gas, sufficient to bring the bed particles into rapid motion, has been supplied. In this case, “uniform fluidization” means that, once fully-fluidized, the particles in the bed are in universal, random motion. Mixing and internal heat transfer within a fully-fluidized bed are both very rapid, and a relatively-shallow bed can often be operated in a nearly-isothermal manner, meaning that the temperature at any point within the bed is almost completely uniform.
Fluidized beds may be adversely affected by a phenomenon referred to as “slugging.” Slugging develops in beds that have L/D ratios greater than 1.5-2.0 and fluidized beds composed of particles larger than a few hundred microns are especially prone to slugging. Slugging is a phenomenon in which a gas-filled bubble forms in the bed, and the diameter of the bubble rapidly expands to reach the full diameter of the bed. Then the entire bed above the bubble begins to move upward as a coherent body (a “slug”), with very little relative motion between particles in the “slug.” The slug can rise for many bed diameters before the cohesion of the slug begins to break down, and the particles in the slug then drop rapidly back down toward the lower levels of the reactor. Usually, the bubble forms at an elevation of 1.5-2.0 reactor diameters above the bottom of the bed. While the slug is rising, a region of well-fluidized bed material can be observed in the lowest parts of the bed, with an open space, containing only the fluidizing gas, appearing between the top of the well-fluidized region and the bottom of the coherent slug. As the slug disintegrates, the bed material from the slug drops down onto the bed material in the lowest parts of the bed, suppressing fluidization until the bubble re-forms and the next slug is lifted. Slugging is usually cyclic or periodic, and, once it begins, it can continue with regularity until it is interrupted by a change in operating conditions. Slugging can also be affected by the properties of the bed material. Two beds, of equal depths and bulk densities, may behave very differently if the particle size distribution is different, or the sphericity of the particles in either bed is changed.
Slugging is undesirable for several reasons. Most importantly, when slugging occurs, longitudinal mixing in the bed is retarded, and particles from the highest points in the bed move very slowly down toward the bottom of the bed (and vice versa). The uniformity of axial temperature is thereby compromised, and considerable gradients in temperature can be observed along the height of the bed. Slugging also creates cyclical stresses on the walls and floor of the bed, particularly if the bed is disposed within a reactor, and the effect of cyclic loading and unloading on the reactor support structure, and the concomitant effect on process chemistry, can destroy any semblance of process uniformity. The vibration, or cyclical loading, of the reactor walls and support structure, can lead to mechanical failures, and the variation in the process chemistry will also make it impossible to operate with a useful level of process control. Slugging may also significantly increase the attrition of particles that comprise the fluidized bed, because the large-amplitude, cyclical motion of the bed tends to involve the bed particles in more energetic collisions with other particles and with the walls of the vessel within which the bed is contained.
As mentioned above, the problem of slugging can generally be avoided simply by using a shallower bed or, in some cases, using particles of smaller diameters. However, there are applications where a shallow bed is simply not practical. If the bed has catalytic properties that are essential to the process chemistry, then the weight of catalyst in the fluidized bed may need to be above some threshold, relative to the mass flow rate of vapors passing through the bed, in order for the desired reactions to occur. In the case of the present invention, the desired deoxygenation reactions that are required to carry out effective hydropyrolysis cannot be carried out in a shallow fluidized bed of catalyst. If the bed is too shallow, the vapors will exit the bed before the desired effect is achieved. The mass flow rate of fluidizing gas required to fluidize a bed also depends on the diameter of the bed. In some situations, particularly in pressurized reactors, the diameter of the bed must be held below a certain value, so that a gas velocity sufficient to fluidize the bed can be achieved with the available mass flow rate of fluidizing gas. The process of the present invention, as described below, preferably includes the use of a deep fluidized bed, composed of relatively large catalyst particles. Because this bed is inherently prone to slugging we have incorporated in this invention a means of curtailing slugging. Slugging is avoided or controlled via the use of an insert or other anti-slugging modification of the hydropyrolysis reactor, which is disposed within the fluidized bed. The design and application of the insert within the reactor or other modifications of the hydropyrolysis reactor to inhibit slugging are important aspects of the invention. The use of the insert or other anti-slugging modification of the hydropyrolysis reactor makes it possible for the fluidized bed to maintain proper fluidization and be of the required depth to carry out the desired hydropyrolysis reactions. The insert further makes it possible for the bed to be composed of relatively-large catalyst particles, which are large enough to be retained in the bed while smaller particles of solid residue (char) are elutriated and carried out of the bed within the gaseous product stream.
The behavior of a fluidized bed will vary depending on the flow rate of fluidizing gas passing through the bed. The process of the present invention, as described below, specifically involves a bubbling fluidized bed. In a bubbling fluidized bed, a flow rate of fluidizing gas is supplied that is sufficient to vigorously agitate and mix the bed, and is large enough that open voids, containing almost exclusively fluidizing gas, are formed. However, the flow rate is not large enough to entrain the solid catalyst particles from which the bed is composed in the gaseous exhaust stream and permanently separate them from the bed.